1. Field of the Invention
The present invention relates generally to reference signal generation in a cellular mobile communication system, and more particularly, to efficient reference signal generation in a cellular mobile communication system based on a Distributed Antenna System (DAS).
2. Description of the Related Art
Mobile communication systems have evolved into a high-speed, high-quality wireless packet data communication systems that provide data and multimedia services that are far beyond the early voice-oriented services. Various mobile communication standards, such as, for example, High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in Institute of Electrical and Electronic Engineers (IEEE), have been developed to support the high-speed, high-quality wireless packet data communication services. Particularly, LTE is a most promising technology that is capable of facilitating the high speed packet data transmission and maximizing the throughput of the radio communication system with various radio access technologies. LTE-A is the evolved version of LTE, which was developed to improve the data transmission capability.
The existing 3rd generation wireless packet data communication systems, such as, for example, HSDPA, HSUPA and HRPD, use technologies such as Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling (CSS) to improve the transmission efficiency. With the use of AMC, a transmitter can adjust the amount of transmission data according to the channel state. Specifically, when the channel state is not ‘Good’, the transmitter reduces the amount of transmission data to adjust the reception error probability to a desired level. When the channel state is ‘Good’, the transmitter increases the amount of transmission data to adjust the reception error probability to the desired level, thereby efficiently transmitting a large volume of information. With the use of a Channel-Sensitive Scheduling-based resource management method, the transmitter selectively services the user having a better channel state among several users, thus, increasing system capacity as compared to a method of allocating a channel to one user and servicing the user with the allocated channel. Such a capacity increase is referred to as ‘multi-user diversity gain’. In sum, the AMC technique and the Channel-Sensitive Scheduling method are each a method of applying an appropriate modulation and coding scheme at a most-efficient time determined depending on partial channel state information fed back from a receiver.
When used with Multiple Input and Multiple Output (MIMO), the AMC technique can be used to determine a number of spatial layers for transmission or rank. In this case, the AMC scheme is implemented in consideration of the number of layers to be used in MIMO transmission as well as the coding rate and modulation level.
There has been much research conducted to adopt Orthogonal Frequency Division Multiple Access (OFDMA) to next generation communication systems in place of Code Division Multiple Access (CDMA), which has been used in 2nd and 3rd Generation mobile communication systems. The standardization organizations such as 3GPP, 3GPP2, and IEEE are developing standards for an enhanced system based on the OFDMA or modified OFDMA. OFDMA promises to increase system capacity as compared to CDMA. One of the factors affecting the increase in system capacity in an OFDMA system is the use of frequency domain scheduling. As a channel sensitive scheduling technique uses a time-varying channel for capacity gain, it is possible to increase the capacity gain with a frequency-varying channel characteristic.
The conventional cellular communication system is composed of a plurality of cells, as shown in FIG. 1, to provide mobile communication services with the above-described techniques. FIG. 1 is a diagram illustrating a cellular mobile communication system composed of three cells, each centered around a transmit/receive antenna.
Referring to FIG. 1, the mobile communication system includes a plurality of cells 100, 110, and 120, each centered around an antenna 130, and first and second User Equipment (UEs) 140 and 150. The eNB serves the first and second UEs 140 and 150 within the cells 100, 110, and 120 to provide a mobile communication service. Within the cell defined as the service area of the eNB using the antenna 130, the first UE 140 is served at relatively low data rate as compared to the second UE 150, since the first UE 140 is more distant from the antenna 130 than the second UE 150.
As shown in FIG. 1, the formation of the antenna arranged at the center of a cell is referred to as Central Antenna System (CAS) in mobile communication systems. In the case of a CAS, even when multiple antennas are provided, all of these antennas are arranged at the center of the cell to define the service area. In a mobile communication system implemented with the CAS-based antenna formation as shown in FIG. 1, each eNB has to transmit reference signals for the UE to measure downlink channel state and modulate downlink signals. In the case of 3GPP LTE-A, the UE estimates a channel with a Demodulation Reference, Signal (DM-RS) and measures a channel state between the eNB and the UE based on Channel Status Information Reference Signal (CSI-RS). The DM-RS and CSI-RS are transmitted by eNB.
FIG. 2 is a diagram illustrating a downlink reference signal structure with DM-RS and CSI-RS transmitted from eNB to UE in an LTE-A system.
In FIG. 2, reference numbers 220 and 221 denote locations of DM-RS for four antenna ports, respectively. Specifically, the DM-RS for ports 7, 8, 11, and 13 are transmitted at the locations denoted by reference number 220, and the DM-RS for ports 9, 10, 12, and 14 are transmitted at the locations denoted by reference number 221. In order to discriminate among the DM-RSs mapped to the same location, Code Division Multiplexing (CDM) is used with the codes assigned to the respective ports as shown in Table 1 below.
TABLE 1Antenna port p[ wp(0) wp(1) wp(2) wp(3)]7[+1 +1 +1 +1]8[+1 −1 +1 −1]9[+1 +1 +1 +1]10[+1 −1 +1 −1]11[+1 +1 −1 −1]12[−1 −1 +1 +1]13[+1 −1 −1 +1]14[−1 +1 +1 −1]
The DM-RS number at the positions for DM-RS ports is defined by Equation (1), as set forth below:
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ⁢                                        ⁢                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ⁢                                        ⁢                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              12            ⁢                                                  ⁢                          N              RB                              max                ,                DL                                              -          1                                    (        1        )            
In Equation (1), NRBmax,DL denotes a number of Resource Blocks (RB) available in downlink, m denotes the sequence index, and the sequence indices of 12 DM-RS resources per RB for the DM-RS ports are mapped in a frequency preference manner as shown in FIG. 3. c(i) denotes a pseudo-random sequence having an initial value denoted by Equation (2) below:cinit=([ns/2]+1)E(2NIDcell+1)E216+nSCID  (2)where NIDcell denotes a cell ID, and nSCID denotes scrambling identity information provided by a scrambling identity field of DCI format 2b or 2C transmitted in PDCCH which is set to 0 or 1. Since the antenna ports of the same cell have the same cell ID, the DM-RS sequences of the antenna ports are discriminated by nSCID.
Referring again to FIG. 2, the signals for two CSI-RS antenna ports can be mapped to the locations denoted by reference numbers 200 to 219, respectively. Specifically, the eNB transmits the CSI-RSs for two antenna ports at a location denoted by reference number 200 in order for the UE to measure downlink. In the case of the cellular system having multiple cells as shown in FIG. 2, CSI-RSs can be transmitted in different locations. For example, a CSI-RS for the cell 100 can be transmitted at a location denoted by the reference number 200, a CSI-RS for the cell 110 at a location denoted by the reference number 205, and a CSI-RS for the cell 120 at location denoted by the reference number 210. By allocating different time-frequency resources, it is possible to avoid interference among the CSI-RSs for different cells.
The CSI-RS sequence transmitted at a location mapped for a CSI-RS port is defined by Equation (3) below:
                                                        r                              i                ,                n                ,                                      ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ⁢                                        ⁢                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ⁢                                        ⁢                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              N            RB                          max              ,              DL                                -          1                                    (        3        )            
In Equation (3), c(i) denotes a pseudo-random sequence of which an initial value is defined by Equation (4) below:cinit=210·(7·(ns+1)+l+1)·(2·NIDcell+1)+2·NIDcell+NCP  (4)
In Equation (4), l denotes an OFDM symbol order in a slot, NCP denotes the length of Cyclic Prefix (CP) for use in the corresponding cell and which is set to 0 or 1.
In the case of the CAS-based system as shown in FIG. 1, the transmit/receive antennas of an eNB are arranged at the center of the cell, such that the communication service to the UE located far from the cell's center is limited in its data rate. Thus, high data rate service is limited to the UEs located near or around the center of the cell in the CAS-based system. Specifically, the conventional cellular mobile communication system has a drawback in that the UEs located at the cell boundary cannot be served at a high data rate, while the UEs located near the cell's center can be served in this manner.